Researchers found metformin use alone could statistically predict patients' likelihood of surviving five years.

Metformin, an inexpensive and common diabetes drug, may fight ovarian cancer, according to a new study.

Women being treated for ovarian cancer at the Mayo Clinic who were also taking metformin for their diabetes lived longer than their counterparts who had the same stage of cancer, but were not diabetic and not taking metformin, the study showed.

Among the 239 women in the study with epithelial ovarian cancer, 67 percent of those taking metformin were still alive five years after their diagnosis, compared to 47 percent of the 178 women not taking metformin. All of the patients in the study underwent the same chemotherapy treatments.

Although it is not yet clear how the drug may work against cancer, it's probable that "metformin results in starving the cancer cells of their energy source — namely glucose — inhibiting the growth of these cells," said study researcher Viji Shridhar.

Only metformin, not other anti-diabetic drugs, demonstrated a survival advantage for ovarian cancer patients in this study.

Analysis of a more heterogeneous group of ovarian cancer patients had a similar result: 73 percent of the 72 metformin-taking patients were alive at five years, while 44 percent of the 143 patients not taking metformin lived that long.

"I think there is good rationale to support further study of metformin," in breast cancer, ovarian cancer and endometrial cancers, said Dr. Robert L. Coleman, gynecological oncologist at the MD Anderson Cancer Center in Houston.

At this point, researchers have gathered enough data to show that it's very likely that metformin has anti-cancer activity, said Michael Birrer, an oncologist at the Dana-Farber Cancer in Boston who was not involved in the study.

"But all of [the data] is retrospective so for, including this study," Birrer said. In other words, the study showed an association, rather than a direct cause-effect link between metformin and ovarian cancer survival rates.

Birrer added that he treats ovarian cancer patients, and some have requested metformin prescriptions, but he has to refuse — until a prospective, randomized trial is conducted and shows benefit, cancer patients cannot be given metformin for the purpose of treating their cancer.

Shridhar said he and colleagues are currently laying the groundwork for such a study.

In the current study, the researchers found metformin use alone could statistically predict patients' likelihood of surviving five years, independent of how far along the patient's ovarian cancer was, the cancer's aggressiveness, and body mass index — all of which are known to affect long-term survival.

Ovarian cancer is an uncommon, but particularly deadly disease—about 22,000 U.S. women will be diagnosed with the disease this year, and more than 15,000 women will die of ovarian cancer in 2012, according to National Cancer Institute estimates. [5 Things Women Should Know About Ovarian Cancer]

While the idea that metformin may fight cancer is not entirely new, this is the largest case study to analyze the potential of metformin for ovarian cancer.

Laboratory research has shown that metformin has anti-cancer properties, and a a clinical trial is now testing whether giving metformin to women with early-stage breast cancer can help prevent progression to later-stage disease.

Another open question is whether metformin would be effective on both late-stage and early-stage ovarian cancer. "Since metformin alters the cancer cell's metabolism, we anticipate that it will be effective in reducing tumor growth of early-stage tumors," Shridhar said.